1. Field of the Invention
The present invention relates to a sulfuric acid electrolytic cell which forms a solution containing chemical species of oxidation nature through electrolysis of sulfuric acid and a sulfuric acid recycle type cleaning system applying the sulfuric acid electrolytic cell.
2. Description of the Related Art
It is reported that electrolyzed water with oxidation nature or reduction nature being formed through water electrolysis can be utilized in various areas including medical and foods industries.
Also, in the cleaning process of electronics components, cleaning with electrolyzed water commands attention for its less danger in storage or transportation available by the on-site type, and possibility for reducing costs of wastewater treatment, as well.
On the other hand, in the wet cleaning technology applied for washing silicon wafer work such as in the manufacture of semiconductor devices, removal of used photoresist, metals and organic contaminants is commonly performed by SPM in which cleaning is carried out with mixed liquid of sulfuric acid and hydrogen peroxide.
Removal effect of SPM is reported to be deriving from strong oxidizing power of persulfuric acid and mixing heat generated through oxidation while sulfuric acid is mixed with hydrogen peroxide.
Persulfuric acid or persulfate is known to be formed through electrolytic oxidation of sulfuric acid, and has been already electrolytically manufactured on an industrial scale; for instance, acidic sulfate like ammonium persulfate (NH4)2S2O8 has been manufactured by anodic oxidation of ammonium sulfate (NH4)2SO4.
The inventors of the present invention have invented the manufacturing method of persulfuric acid by an electrolytic process using a conductive diamond electrode and a cleaning process, as technologies which supply, continuously and quantitatively at a high efficiency, persulfuric acid with a high cleaning effect, and filed for patent (Patent Document 1).
This kind of conductive diamond electrode, which gives larger oxygen over potential, compared with platinum electrodes widely used as electrodes to form persulfate, shows a higher efficiency in electrolytic oxidation of sulfuric acid into persulfuric acid and also a higher chemical stability and a longer electrode life.
In the semiconductor cleaning process, it is essential for the cleaning objects not to be contaminated in view of a higher yield of the products and therefore, chemical solutions whose cleaning effect is stringently controlled are applied. Impurities in sulfuric acid available in the market for the semiconductor manufacturing process have been controlled on the ppb level; however persulfate produced on an industrial level contains impurities as much as over 1000 times, cleanliness level of which, therefore, does not meet at all the level for the semiconductor cleaning process.
The conductive diamond electrode has a construction in which a conductive diamond film is formed as a thin film at a few to hundreds micron in thickness on a conductive substrate with a smooth surface. Electrolytic current supplied via the substrate causes electrochemical reaction at the surface of the conductive diamond film. Although its chemical stability is intrinsic, the mechanical strength of the conductive diamond film, being a thin film, depends on the strength of the substrate.
A conductive diamond film is generally formed by means of CVD method. Film forming is performed in a hydrogen based atmosphere, at over 1000 degrees Celsius; therefore, the substrate is required to withstand these environments with no volume change from corrosion or phase transition. Also, to secure a good adherence between the conductive diamond film and the substrate surface, the coefficient of thermal expansion of the conductive diamond film must be close to that of the substrate.
As the substrate material for a conductive diamond electrode, single-crystal silicon or polycrystalline silicon plate material is used because of said reason, which is generally conductive, has a similar coefficient of thermal expansion to diamond and chemically stable, though such materials as niobium or titanium, superior in corrosion resistance, may sometimes be used.
Electrolytic cells applying conductive diamond electrodes have conventionally been suggested. Mainly applied for the production of chemical substances or water treatment, this type of electrolytic cells with two electrodes arranged oppositely has functions to cause electrochemical reactions on the electrode surface with direct current supplied between electrodes while electrolyte is being supplied to the electrolytic chambers, separated by a diaphragm, of both electrodes.
When the electrolytic cells applying an electrode with a conductive diamond film formed on a silicon substrate is operated, various phenomena occur on the electrode, in addition to electrochemical reaction, including Joule heat generation by electrolytic current, pressure loading by solution supplied, compression by gaskets or O-rings if used for liquid sealing, chemical change in materials such as corrosion through contact with the electrolyte. Conductive diamond electrodes are required to have resistivity against these phenomena.
Under the operating conditions of a conductive diamond electrode, the electrolytic current density is usually below 100 A/dm2 or even at maximum, below 300 A/dm2. Therefore, Joule heat generated under such conditions is presumed below the temperature at which the diamond film is formed, and therefore, as far as Joule heat generated by electrolytic current is removed by electrolyte, any temperature as high as the conductive diamond film and silicon substrate deteriorate will not emerge. However, on the surface of the current collector where electrolytic current is supplied by contacting the substrate, formation of oxide film may be accelerated by Joule heat and contact resistance may be increased with time. Accordingly, such contact is necessary that adherence and conductivity between the current collector and the silicon substrate do not deteriorate even under generation of Joule heat and also no quality change with time occur at the surface of electric feed material by oxidation, etc.
Pressure of feed solution is built up by the liquid supply pump which feeds solution to the electrolytic cells. The pressure increases with increase of the solution amount to be treated, and approximately 0.4 MPa, at maximum, of solution pressure is expected as auxiliary equipment to the cleaning system used in the manufacture of semiconductor device. This pressure is directly applied to the electrode surface, and if deflectable stress is loaded on the electrode, overall cracking may occur.
Tightening compression of gaskets or o-rings is a design pressure required for the electrolytic cells, which is worked out from the volume of feed solution or the pressure by feed solution. Silicon is a material which is relatively hard, but brittle, having cleavage and therefore, the silicon substrate tends to be wholly destroyed once break occurs. This is why this material is better not to be used where impact or stress are applied. In particular, when there is a freely moving end and a supporting point on the silicon substrate, any break or crack occurring from a part of the silicon substrate could develop to a wholly breakage. If the breakage spreads over the entire silicon substrate, and if liquid-sealing by gaskets or o-rings is provided around the periphery of the conductive diamond electrode, the breakage allows electrolyte to leak outside the sealing structure.
As an example of the conventional electrolytic cells applying conductive diamond electrodes, such design is disclosed that disc-shaped diamond electrodes are arranged face to face, being supported by conductive supporting discs, through which electricity is supplied. (Patent Document 2)
In Patent Document 2, the diamond electrode is closely attached to the conductive supporting disc, but as a weak point, pressing force, which makes the conductive supporting disc functioning as sealing part attach to an o-ring seal and a supply washer, is applied by thrust of coil springs and by clamping torque of through bolts. Such pressing force on the electrode surface tends to be uneven because of multiple numbers of coil springs, causing breakage of electrodes or leakage of liquid easily, while, on the other hand, local adjustment is available spot-wise on the electrode surface.
If the electrode attached to the conductive supporting disc is considered as an integral unit, an entire breakage of the electrode is hard to occur when the supporting disc is thick enough, but still breakage is easy to occur where pressing force is locally large. Therefore, the construction of this electrolytic cell has drawback in terms of easy breaking and cracking.
Also, in Patent Document 3, such construction is disclosed that liquid allows to flow without producing short-circuit or passive part on the round-shape electrode, but any description is given neither to diaphragm nor to mechanical consideration to cope with electrode cracking.
Patent Document 4, which is revised version of Patent Documents 2 and 3, does not relate to the construction having diaphragms, and applies elastic conductive metal fibers, etc, having elasticity, being not rigid, softer compared with silicon for the power feeding to the electrode, and therefore when electrodes are subject to a strong pressure by feed solution, etc., breakage by deflection is easy to occur on the entire silicon, since the substrate and current collector do not form an integral unit.
Generally speaking, in case of plate material, thicker is stronger to flexure, and in case of two sheets of plate, integral construction is stronger to flexure. In Patent Document 4, pressing force to the current collector is applied by multiple numbers of springs and conductive metal fiber construction having elasticity, as with the method by Patent Document 2, and therefore, similar problems to Patent Document 2 could occur. Whereas, the specification explains that in order to obtain uniform pressing force, conductive metal fibers are employed; however, in fact, conductive metal fibers have elasticity, which allows significant change in thickness by pressing force and therefore, it should be difficult to compress the large area to a uniform thickness. In view of this, the displacement in thickness with the electrolytic cells described in Patent Document 4 will cause deflection, which will lead to generation of cracking. In addition, compression deformation of conductive metal fibers generates resistance by conductive metal fibers themselves or distribution of contact resistance by conductive metal fibers with diamond electrodes or with the current collector, which leads to uneven distribution of electrolytic current or Joule heat, eventually causing local loading on electrodes or electrolytic cells or variation in performance.
When electrolysis products are obtained through electrolysis of sulfuric acid, oxidizing substance is formed at the anode through electrolytic oxidation of substance contained in electrolyte, and reducing substance is formed at the cathode through electrolytic reduction of substance contained in electrolyte. If these two kinds of materials are made contacted, each substance may return to its initial substance through oxidation and reduction, and therefore, if no diaphragm exists in the electrolytic cells, as with afore-mentioned Patent Documents 3 and 4, it is considered that electrolytic products, after electrolysis, contained in the electrolyte, immingle and are subjected to oxidation and reduction reactions with each other to return to respective initial substances or form different substances from electrolytic products, and are drained outside the electrolytic cells. This type of electrolytic cells using no diaphragm will achieve inefficient performance as a reaction system.    [Patent Document 1] Tokkai 2007-332441 Patent Gazette    [Patent Document 2] Tokkai 2004-525765 Patent Gazette    [Patent Document 3] Tokkai 2006-225694 Patent Gazette    [Patent Document 4] Tokkai 2007-262531 Patent Gazette